1. Field of the Invention
This invention relates to an Al alloy extruded square pipe for use in the structure of an automobile, and more particularly, to an Al alloy extruded square pipe which is an energy absorption member excellent in axially compressive properties in the lengthwise direction for an automobile front side member.
2. Description of the Prior Art
A member for an automobile front side member (which will be hereinafter simply referred to as also a front side member) as shown by a front side member 150 in FIG. 15 is provided on the left and right sides of an engine room at the front of an automobile, and functions as an absorber which absorbs energy of the shock of impact when happened, resulting in maintaining the safety of users.
As generally known, when the front side member 150 causes multiple buckling deformation so as to be crushed (i.e., so-called accordion buckling) as the result of receiving a compressive load in the lengthwise direction caused by the impact, the buckled front side member absorbs energy more efficiently than the broken or bent front side member.
FIG. 16 shows the relation between the load and the rate of deformation when a compressive load is axially applied to the front side member 150. As shown in FIG. 16, the front side member 150 causes buckling at an initial load P1, and makes rapid progress in deformation at a fixed load (mean load) P2, so that multiple buckling deformation is presented. The initial load P1 is considered to be as low as possible with the intention of reducing the shock of impact when happened, while the fixed load P2 is considered to be as high as possible with the intention of improving energy absorption efficiency.
A front side member is required to be lightweight, be easily formed, be easily installed with various members such as a suspension and an engine and so on.
Conventionally, various types of front side members have been proposed as follows. As shown FIG. 10, for instance, proposed has been a front side member 60 formed by cutting an Al alloy extruded square pipe 61 crosswise into a plurality of members 62 and thereafter integrating these members with each other again by welding, whereby energy of the impact is absorbed by deforming the front side member 60 so as to start from its weld beads 63 (See Japanese Patent Toku-Kai-Sho No. 64-67482). However, the problems with this method are that the manufacturing process is increased in number, and its cost goes up, since this front side member is structured by integrating a large number of members with each other by welding.
As shown in FIG. 11, proposed has been a front side member 70, in which members 71, 72 formed by sheet metal processing are welded together into a member approximately square in sectional shape as shown in FIG. 11(a), and an intermediate portion is formed to be pentagonal or more polygonal in sectional shape as shown in FIG. 11(b) (See Japanese Patent Toku-Kai-Hei No. 6-206576). Otherwise, as shown in FIG. 12, proposed has been a front side member 80, in which concave and convex beads 82, 83 are formed at an intermediate portion of an Al alloy extruded square pipe 81 by pressing (See Japanese Patent Toku-Kai-Hei No. 7-52820). The front side member 70 or 80 still presents the problems in that the manufacturing process is increased in number and its cost goes up.
On the other hand, a front side member annular in sectional shape has been also proposed for the purpose of heightening energy absorption efficiency (See Japanese Utility Model Jitsu-Kai-Hei No. 4-108484). However, since this front side member is annular in sectional shape and has no flat portion, it is difficult to mount other members on this front side member.
As shown in FIG. 13, proposed has been an Al alloy extruded square pipe 90, in which all four corner portions 92A to 92D and all four side portions 93A to 93D are uniform in wall thickness, for the purpose of reducing the weight or the like. However, this square pipe is still inferior in energy absorption efficiency.
As shown in FIG. 14, proposed has been a front side member 100 which is a rectangular pipe having the shape, in which a cross is enclosed by a rectangle in section, and composed of an inner longitudinal pillar 104, an inner lateral pillar 105, two longitudinal sides 103A, 103C of a large wall thickness and two lateral sides 103B, 103D of a small wall thickness, whereby even an elongate member easily causes multiple buckling deformation (See Japanese Patent Toku-Kai-Hei No. 6-247338).
However, the problems with the member 100 are that this member has a complicated sectional shape, is hard to be formed by extrusion, and increases in weight.
3. Object of the Invention
It is an object of the present invention to overcome the above problems, and particularly, to provide a member which is used for an automobile front side member and satisfies the requirements in that a member should have a simple sectional shape, be easily formed by extrusion and easily worked subsequently to extrusion, be lightweight as much as possible, be provided with a flat portion for mounting other parts on the member, be excellent in axially compressive properties in the lengthwise direction against an impact, that is, show high energy absorption efficiency and so on.